The present invention relates generally to machines for forming webs of sheet material and, more particularly, to a method and apparatus for determining cross direction responses and mappings of actuation cells used to control the operation of such machines.
Machines which produce webs of sheet material such as paper, plastic and aluminum, face common process control problems in producing webs which satisfy specifications for the given sheet material. Web specifications commonly include ranges for characteristics of the web including thickness, moisture content, weight per unit area and the like. Quality control is complicated since the specified characteristics vary in both the machine direction (MD) or direction of movement of the web through the machine and in the machine cross direction (CD) or laterally across the web.
The MD variations are generally affected by factors that impact the entire width of the web, such as machine speed, the source of base material being formed into a web by the machine, common supplies of working fluids like steam, and similar factors. CD variations, represented by profiles or profile signals, are normally controlled by arrays of actuation cells distributed across the width of the machine. On paper making machines, for which the present invention is particularly applicable, the CD actuation cells include basis weight actuators which control the slice of a headbox, steam shower nozzles, infrared (IR) heaters which control CD moisture variations, and other known devices.
Adjustment of CD actuation cells generally affects a portion of the profile that is wider than the individual actuation cells. Thus, for controlling the CD profile of a web forming machine, it is important to know which portion of the profile is affected by each CD actuation cell and how the profile is changed by adjustments of each CD actuation cell. The functional relationship that describes which part of the profile is affected by each CD actuation cell is called "mapping" of the CD actuator cells. The functional curve that indicates how the process profile is changed by the adjustment of a CD actuation cell is called the "response" of the CD actuation cell.
Not only does the CD response of an actuator cell typically spread over a much wider area than the area of the cell itself, but also the CD mapping of an actuator cell can vary or shift for different operating conditions. To obtain a desired profile for the web of sheet material being formed, it is essential to have response and mapping information which precisely corresponds to each actuation cell and also to the different operating conditions which the cell may encounter.
In the past, response information typically has been determined on-line by means of feedback arrangements. In these systems, the control signals which are passed to the machine to maintain the web forming process within suitable limits are monitored and compared to a profile signal which is generated by monitoring the web of material within the machine or as it emerges from the machine. Unfortunately, since the control signals must be substantially limited in terms of amplitude excursions, the resulting response and mapping characteristics are not as representative as is desired for accurate control of the machine.
Another system has been described for determining both response and mapping information by interrupting a system controller and then changing the setting of a given actuation cell. The actuation cell is changed a number of times and the resulting changes in the web are monitored. By summing and otherwise processing these changes, the response and mapping for the actuation cell can be determined. Unfortunately, the described system requires a substantial period of time to evaluate one or more series of actuation cells. Further, there is no assurance that the web being produced during the evaluation will be within required specification limits such that the web can be used.
Accordingly, there is a need for improved determination of the response and mapping characteristics of the individual actuation cells of web forming machines. The response and mapping characteristics should be quickly and accurately obtained, even while the machine is operating and without adversely affecting the quality of the web of material being formed. Preferably, any method and/or apparatus for determining the response and mapping characteristics could be inexpensively incorporated into new machines and also retrofitted into existing machines with substantially equal improvements in the operations of the existing machines.